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Effects of polytitanium chloride and polyaluminum chloride pre-treatment on ultrafiltration process: Floc properties and membrane fouling
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Effects of polytitanium chloride and polyaluminum chloride pre-treatment on ultrafiltration process: Floc properties and membrane fouling Xin Huang a,b, Baoyu Gao a,∗, Qinyan Yue a, Yan Wang a, Qian Li a a
School of Environmental Science and Engineering, Shandong University, No. 27 Shanda South Road, Jinan 250100, Shandong, People’s Republic of China Key Laboratory of Drinking Water Science and Technology, Research Center For Eco-environment, Chinese Academy of Science, Beijing 100085, People’s Republic of China
b
a r t i c l e
i n f o
Article history: Received 6 November 2017 Revised 30 March 2018 Accepted 31 March 2018 Available online xxx Keywords: Coagulation–ultrafiltration Membrane fouling Floc properties Polytitanium chloride
a b s t r a c t This study compared the membrane fouling of polytitanium chloride (PTC) and polyaluminum chloride (PAC) in coagulation–ultrafiltration (C–UF) process in real water treatment. According to pre-coagulation experiment, the chosen dosage for PTC and PAC was 18 mg/L as Ti and 8 mg/L as Al, respectively. Floc properties also comparatively studied to further analyze the membrane fouling. PTC formed larger and stronger flocs with higher dimension fractal dimension (Df ) value compared with those formed by PAC. Moreover, PTC C–UF achieved less serious decline of permeate flux during the filtration process. The external membrane resistance of PTC C–UF process was lower than that of PAC C–UF process, while the cake layer of PTC seemed to be more strongly attached on the membrane. For the total internal membrane resistance, PAC achieved lower degree compared with PTC. However, the irreversible membrane resistance of PAC was more serious than that of PTC. Thus, PTC was an effective coagulant for C–UF process. © 2018 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Membrane technology, especially low-pressure membrane including microfiltration and ultrafiltration has been a promising process for drinking water treatment [1]. Membrane fouling has been one of the most serious obstacles which impedes the development of membrane technology [2]. Some pretreatments could be integrated with membrane filtration which contribute to the reducing membrane fouling degree and enhancing the removal of contaminates in water [3]. Coagulation has been one of the most commonly used pretreatment technologies due to its high performance and low cost to remove not only colloids and particles but also natural organic matter (NOM) [4]. The performance of coagulation–ultrafiltration (C–UF) hybrid process usually depends on the features of coagulation, such as coagulant type, dosage and method of mixing and so on [5–7]. Previous study suggested that membrane fouling could be controlled by inorganic coagulant, while it would be aggravated by polymeric organic coagulant [6]. They also suggested that polyaluminum chloride (PAC) exhibited the good performance in reduction of membrane fouling.
∗
Corresponding author. E-mail addresses: [email protected], [email protected], [email protected] (B. Gao).
In addition, PAC has been one of the most popular coagulants in the application of water treatment due to its high performance and low cost for recent decades. However, residual aluminum in drinking water still imposes some potential neuropathological hazards on human being and other organisms [13,14]. In addition, there is another environmental problem caused by aluminum salts. Large amount of sludge produced after coagulation is difficult to deal with and may result in secondary pollution. Recently, titanium salts have been widely studied in coagulation and flocculation due to its well coagulation performance and sludge reuse possibility [15–17]. Shon et al. have first found that sludge of TiCl4 coagulation could be used to produce photocatalyst TiO2 which even performed better photoactivity than commercial P-25 TiO2 [17]. Afterwards, research on Ti-based coagulants gradually increase. To avoid the lower pH of the titanium coagulation effluent, Zhao et al. have successfully synthesized inorganic polymeric titanium coagulantpolytitanium chloride (PTC) which has been proved to be an effective and promising coagulant for water treatment [16]. Zhao et al. have also compared coagulation performance and floc properties of PAC and PTC with various basic values in humic–kaolin water treatment [15]. Floc properties, including floc size, floc strength and floc density also believed to significantly affect membrane fouling, especially the cake layer. Cake resistance has also been proved to be the primary account for the permeate flux decline and membrane
https://doi.org/10.1016/j.jtice.2018.03.057 1876-1070/© 2018 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: X. Huang et al., Effects of polytitanium chloride and polyaluminum chloride pre-treatment on ultrafiltration process: Floc properties and membrane fouling, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.03.057
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fouling [8,9]. Waite et al. have found that larger floc with loose structure would form a more permeable cake layer [10]. Yu et al. proved floc breakage and re-growth properties also influence the trans-membrane pressure [8]. The membrane fouling or resistance would significantly depend on the physicochemical properties and structure characters of aggregates in the feed water [11,12]. Floc properties is much related to coagulation mechanism which will also affect the membrane fouling. Moreover, floc strength and recoverability are also due to it is inevitable for flocs to break into small particles which generally reduce the solid/liquid separation efficiency and may also plug membrane pores. Previous study also found that compared to flocs formed by PAC, flocs formed by PTC are larger, stronger and difficult to recover after exposure to high shear rate. However, the application of PTC in coagulation–ultrafiltration (C–UF) process has never been studied before. Therefore, this study compared the performance of PAC and PTC in the application of C–UF process for real water treatment. Coagulation performance and floc properties including floc size, floc size distribution, floc breakage and re-growth factor and floc fractal dimension of PAC and PTC were comparatively investigated. Membrane filtration was conducted by the coagulation effluent without sedimentation. Membrane fouling was also divided into external fouling and internal fouling to further compare the effect of PAC and PTC coagulation on the ultrafiltration membrane resistance. 2. Materials and methods 2.1. Coagulants In this research, PAC/PTC was prepared by slow injection of Na2 CO3 / NaOH solution into AlCl3 /TiCl4 solution (20 %wt) at room temperature. The amount of Na2 CO3 or NaOH varied with the target basicity value ([OH]/[Ti] molar ratio, B value). PAC and PTC with B value of 2.0 and 0.5 were chosen, respectively. 2.2. Test water Test water used in this study was collected from the Queshan Reservoir (located in Jinan, China). The properties of the test water were shown as follows: UV254 = 0.045 ± 0.002, DOC = 2.5 ± 0.2 mg/L, pH = 8.40 ± 0.06, zeta potential = −14 ± 0.3 mV, turbidity = 2.5 ± 0.8 NTU. 2.3. Jar test Jar test was conducted by a porogram-controlled jar test apparatus (ZR4-6, Zhongrun Water Industry Technology Development Co. Ltd., China) which has six 1.5 L square beakers. In each beaker, there is a 50 mm × 40 mm flat paddle impeller. Firstly, 1 L of test water was filled in each beaker. Pre-determined amount of coagulant was added and simultaneously the coagulation program began. Then, each beaker of test water was first stirred at 200 rpm for 1.5 min, followed by 15 min of slow stirring at 40 rpm, and finally settled for 30 min or 20 min for PAC or PTC, respectively. About 150 mL of water samples were collected from 2.0 cm below the solution surface for following measurements immediately at the end of the coagulation process. Unfiltered water samples were applied for turbidity (2100P turbidimeter (Hach, USA)) and zeta potential (Zetasizer 30 0 0HSa (Malvern Instruments, UK)). Filtered water samples (through 0.45 μm glass filter paper) were applied for UV245 absorbance and DOC measurement using Precision Scientific Instrument (Co. Ltd., Shanghai, China) and TOCVCPH (Shimadzu, Japan), respectively. The effluent organic matter was also characterized by luminescence spectrometry (F-4500
FL spectrophotometer, Hitachi, Japan) and performance size exclusion chromatography (HPSEC) method and detected by high performance liquid chromatography system (Waters 1525, Waters, USA). 2.4. Floc formation, breakage and re-growth Different from the jar test program, after the 15 min slow stirring of 40 rpm, the suspension was exposed to a high shear rate (200 rpm) for 5 min, and then another 15 min of slow stirring at 40 rpm was re-introduced for floc to re-grow. Mastersizer20 0 0 (Malvern, UK) was used to measure the dynamic floc size during the whole process. The suspension was monitored by drawing water through the test cell of Mastersizer and back into the jar by a peristaltic pump (LEAD-1, Longer Precision Pump, China) on a return tube with 5 mm internal diameter at a flow rate of 2.0 L/h. The inflow and outflow tubes were placed opposite one another at a depth just above the impeller in the holding ports. Size distribution measurements were taken every 0.5 min and logged onto computer. Floc size, breakage and re-growth factor, fractal dimension were calculated according to Ref. [18] and more details could be found in SI information. 2.5. Coagulation–ultrafiltration procedure To compare the membrane fouling degree of PAC and PTC C– UF process, ultrafiltration experiments were conducted using a 300 mL capacity stirring dead-end batch unit. There was an opening designed for pre-treated samples addition at the top of the cell. The effluent after coagulation without sedimentation was gently transferred from coagulation tank to the dead-end filtration unit and filtered through ultrafiltration membrane. Slow agitation was applied to make a uniform suspension. A constant pressure at 0.80 MPa was maintained by nitrogen during the entire filtration process. In addition, cumulative permeate mass was recorded by an electronic balance (MSU5201S-0 0 0-D0; Sartorius AG, Germany) at every 10 s. The UF membrane (Mosu, China) used in this study was polyethersulfone (PES) flat sheet membrane with molecular weight cutoff (MWCO) of 100 kDa (purchased from EMD Millipore Corporation (CAT. NO. PLHK07610). In addition, the membrane had been soaked in deionized water at least 24 h before use. Modified fouling index (MFI) was calculated and specific method could be found in SI Section 2. The initial membrane flux was in a range of 429–435 L/m2 h. In order to analysis the membrane resistance in detail, membrane resistance was specifically divided into external fouling resistance (Ref , also be commonly called cake layer resistance) and internal resistance (Rif ) according to Ref. [19]. Ref was distributed into loosely and strongly attached external fouling (Ref-l and Ref-s ) according to its immunity to shear force. Meanwhile, internal fouling resistance was expressed as reversible and irreversible internal fouling resistance (Rif-r and Rirr ) based on backwash treatment. The calculation of fouling resistance is described in SI Section 2. 3. Results and discussion 3.1. Pre-coagulation by PTC and PAC The effect of coagulant dosage on coagulation performance of PAC and PTC was comparatively studied in real water treatment and results are shown in Fig. 1. To obtain a further view of coagulation behavior of PAC and PTC, wide dosage ranges (2– 30 mg/L as Al for PAC and 2–90 mg/L as Ti for PTC) were adopted in this study. Residual turbidity of PTC firstly gradually decreased and then sharply increased as Ti dosage increased. Moreover, residual turbidity of PAC remained around 0.5 NTU in
Please cite this article as: X. Huang et al., Effects of polytitanium chloride and polyaluminum chloride pre-treatment on ultrafiltration process: Floc properties and membrane fouling, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.03.057
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the studied dosage range. It could be noted that residual turbidity of PAC was lower than that of PTC. UV254 and DOC removal efficiencies of PAC and PTC both first increased and then slightly decreased with dosage increased. Initially, UV254 and DOC removal efficiencies of PTC were lower than those of PAC. Afterwards, when dosage was over 24 mg/L, UV254 and DOC removal efficiency of PAC slightly decreased as dosage increased, and those of PTC still gradually increased to 69.33% and 36.24%, respectively. The maximum UV254 and DOC removal efficiencies
of PAC were only 58.66% and 36.24%, respectively. The effluent of solution pH is shown in Fig. 1(e). It could be noted that the effluent pH of PTC coagulation was lower than that of PAC. As for PAC and PTC dosage was 8 and 18 mg/L, respectively, the properties of effluent organic matters were characterized by EEM and HPSEC to evaluated their species and molecular weight and shown in Fig. 2. The EEM spectrogram was divided into five parts according to Ref. [20] and each part represent a kind of organic
Please cite this article as: X. Huang et al., Effects of polytitanium chloride and polyaluminum chloride pre-treatment on ultrafiltration process: Floc properties and membrane fouling, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.03.057
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matter. It could be noted that the proteins are the major species in effluent obtained after PAC and PTC coagulation and the intensity of PAC coagulation effluent was higher than that of PTC which meant that the PAC coagulation effluent contained more organic matter. The molecular weight of PAC and PTC coagulation was mainly ranged from 10 0 0–80 0 0 Da. Similarly, the UV absorbance of PAC coagulation effluent was stronger than that of PTC. Zeta potentials of PAC and PTC both gradually increased as dosage increased. Zeta potential of PAC was higher than that of PTC. That indicated that the charge neutralization ability of PAC was better than that of PTC, which was consistent with result of Zhao et al. [15]. However, the higher charge neutralization ability of PAC did not result in higher UV254 and DOC removal. It could be inferred that other coagulation mechanism including sweep, complexion, and adsorption and bridge effect may also played important roles in PTC coagulation. It could be noted that residual turbidity of PTC sharply increased when zeta potential increased from negative to positive. That may be caused by that the surface charge of the formed colloids and particles by titanium and pollutants in water was positively charged, and the inter particle repulsion was increased. Therefore, these particles and colloids is difficult to get together to settle down [21]. In the case of PAC, residual turbidity did not show a rapid increased after isoelectric point. Similar result was found by Yang et al. that PAC (b = 2.0) exhibited better turbidity removal and residual turbidity did not reached minimum at isoelectric point and remained low value even if zeta potential turned
to positive [22]. That was to say surface adsorption and sweep coagulation played efficient role in turbidity removal of PAC. As previous study concerned, membrane fouling was lower under lower coagulant dosage rather than higher dosage, and high dosage coagulation pretreatment usually result in irreversible fouling [5]. They also suggested that charge neutralization coagulation performed better than sweep coagulation in alleviating membrane fouling. Thus, on the basis of above consideration and NOM removal performance, 8 mg/L and 18 mg/L were chosen to be applied in following investigation for PAC and PTC, respectively. 3.2. Floc properties of PTC and PAC According to previous study, the decline of low pressure membrane permeation is mainly caused by the deposition and blocking on or inside the membrane [3]. Also, some study suggested not only floc size and structure would affect the cake layer, but also floc strength could make a difference [10]. Thus, floc properties could significantly affect membrane fouling. Floc growth, breakage and regrowth curve of PAC and PTC was monitored by Mastersizer20 0 0 shown in Fig. 3. More specific floc characters are discussed as follows. 3.2.1. Floc size distribution Particle size distributions (PSD) in the steady period of formation stage of PAC and PTC are shown in Fig. 4. The PSD of PAC flocs
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was mainly between 50–10 0 0 μm. As for PTC flocs, PSD was mostly ranging from 40 0–120 0 μm which exhibited a narrower sharp than that of PAC floc. From Fig. 3, the mean size d50 of PTC was larger than that of PTC. The result was consistent with previous study by Zhao et al. [15]. In addition, previous study by Yu et al. found that more electronegative zeta potential was related with a larger floc mean size d50 for the FeCl3 –humic flocs [5]. In this study, the zeta potential value of PTC (18 mg/L) was lower than that of PAC (8 mg/L). Similarly, greater floc d50 value was obtained by PTC floc. Thus, it could be inferred that the PTC primary aggregates could bind more particles and finally formed larger flocs. 3.2.2. Floc breakage and recovery factor The breakage factor (Bf ) and recovery factor (Rf ) of PTC and PAC were calculated by Eqs. (1) and (2) in SI, respectively, and results are shown in Fig. 5. For the breakage factor, PAC flocs attained smaller value compared with PTC flocs. However, PAC flocs had a larger recovery factor than that of PTC flocs. Similar results were obtained by Zhao et al. who compared PAC and PTC with various B values in synthetic humic acid–kaolin water treatment [15]. They also suggested that PAC coagulants produced flocs with better recoverability than PTC. In the view of breakage factor, PTC floc seemed to be more resistant to high shear rate. That was to say that the inter-particle bind strength inside the PTC flocs was
0
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greater, which may lead to a more active attached and hard to removed cake layer on the surface of membrane. For the recovery ability, PTC flocs performed poorly as previous studies proved [23]. That was to say that the PTC broken flocs were difficult to get together after exposure to high shear rate. Previous studies proved that flocs formed by charge neutralization effect usually have better recovery ability after exposure to high shear stress [9,24]. Compared with PAC, PTC attained a more electronegative zeta potential. Thus, PTC floc showed worse recovery ability. 3.2.3. Floc structure In this study, floc structure was evaluated by Df value calculated by Eq. (4) in SI and results are shown in Fig. 6. Floc Df value generally increased as coagulation proceeding. It was consistent with previous studies that floc Df value was greater after breakage [25]. It was obviously that during the entire process, PTC flocs Df values were all higher than those of PAC flocs. Li et al. had found that fractal dimension of flocs formed by sweep effect was higher than those formed by charge neutralization and bridge effects [26]. Therefore, compared with PTC flocs, PAC flocs seemed to have a more open and looser structure. It could be inferred that the density of the cake layer by PTC C–UF process might be much greater than that formed by PAC C–UF process. As mentioned above, the
Please cite this article as: X. Huang et al., Effects of polytitanium chloride and polyaluminum chloride pre-treatment on ultrafiltration process: Floc properties and membrane fouling, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.03.057
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cake layer of PTC C–UF process was formed by larger, stronger and more compact flocs and that of PAC C–UF process was formed by smaller, weaker and looser flocs. Previous study suggested that larger, looser and more porous flocs attributed to alleviate the decline of flux permeate [10]. However, how the larger and more compact PTC flocs might affect the membrane fouling would be studied in the following section. 3.3. Membrane fouling of PAC and PTC in C–UF application In the membrane filtration experiments, the coagulation suspension without sedimentation was transferred into membrane module and the permeate flux was monitored by an electrical balance and result is illustrated in Fig. 7 and more experiments conditions (including coagulant dose and pH) were conducted as shown in Figs. S1 and S2. The results in Fig. 7, Figs. S1 and S2 showed that the membrane fouling of PTC coagulated water was only a little slighter than that of PAC coagulated water. The permeate fluxes of PAC and PTC coagulated water both first dramatically decreased and then generally decreased relatively gently. The permeate flux decline of PAC was slightly severer than that of PTC and it also could be perceived from MFI shown in Fig. 7. MFI values of PAC and PTC were 99.04 and 81.88, respectively. That was indicated that the total membrane fouling degree was more serious by PAC coagulated water than PTC coagulated water. Moreover, to specifically analyze membrane fouling of PAC and PTC coagulated water, another experiment had been done as described in Section 2.5 and results are shown in Fig. 8. Similarly, the total resistance of PAC C–UF process (3.01∗ 1011 m−1 ) was slightly higher than that of PTC C–UF process (2.66∗ 1011 m−1 ). More specifically, Ref (sum of Ref-l and Ref-s ) (mainly caused by the cake layer deposited on the membrane surface) of PAC C–UF process was higher compared with those of PTC C–UF process. In addition, it could also be note from Fig. 8 that for PAC, Ref-l is higher than that Ref-s ; meanwhile for PTC, Ref-s occupied larger proportion of the external resistance. When comparing PAC C–UF system and PTC C– UF systems, Ref-l of PAC C–UF process was slightly higher; while for Ref-s , PTC C–UF process attained higher value. As for the internal resistance, the total internal resistance (sum of Rirr and Rif-r ) of PAC was lower than that of PTC. However, to be specific, the irreversible resistance Rirr of PAC was slightly higher than that of PAC. For Rif-r , PTC C–UF process was almost three times higher than that of PAC. That was to say, for the internal fouling, PTC C–UF
Fig. 8. Effect of PAC and PTC coagulation pretreatments on ultrafiltration membrane fouling.
resistance was mostly due to the reversible fouling, while PAC C– UF process mainly produced irreversible membrane fouling which was not good for the membrane application. In a comprehensive view of floc properties to analyze the membrane fouling, especially external membrane fouling, membrane fouling has been proved to be related to floc size and structure. In general, larger and looser flocs lead to lower membrane fouling degree. Other study suggested membrane fouling could not only depend on floc size, but also significantly rely on the ability of floc resist to shear stress [27]. They suggested that floc with good ability to resist shear stress would bring out dramatic decline of permeate flux. However, in this study PAC floc was smaller, weaker and looser while PTC floc was larger, stronger but more compact. Therefore, it was not simply to qualitatively analyze the membrane fouling through the floc size and structure. Nevertheless, these floc properties could explain the external membrane resistance. PTC coagulated water achieved lower total membrane fouling and external resistance. In addition, to subdivide and further compare the external fouling, the loosely attached external fouling of PAC was higher but the strongly attached external fouling of PTC was higher. That may due to the high Df value of PTC floc by which a compact cake layer was formed. For the PAC C– UF process, flocs were looser and did not strongly attach on the membrane. Thus, the Ref-l of PAC C–UF process was slightly higher and Ref-s of PTC–UF process was slightly higher. For the internal resistance, pore blocking and adsorption may be one of the most important factors increased membrane fouling. Wang and Tarabara believed that pore blocking mainly contributed to ultrafiltration membrane flux decline [28]. They also suggested that the blocking mechanism could be interpreted by floc size distribution and surface density of membrane. It could be inferred from Fig. 2 that PAC coagulated water may contained more organic matter which was easy to cause membrane pore block. In the membrane of PTC C– UF process, there were not apparent particles. For the irreversible resistance, previous studies suggested that it was mainly caused by the hydrophilic fraction of NOM [29,30]. It could be inferred that the PTC combined more hydrophilic fraction to form larger floc, thus less of hydrophilic fraction was remained in effluent, resulting in lower Rirr value. In addition, the scanning electron microscope (SEM) images of the membrane cross section (after backwashing) were shown in Fig. 9. It was obviously that some small particles were inside the membrane of PAC C–UF process, while the membrane of PTC C–UF process was relatively smooth which
Please cite this article as: X. Huang et al., Effects of polytitanium chloride and polyaluminum chloride pre-treatment on ultrafiltration process: Floc properties and membrane fouling, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.03.057
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Fig. 9. SEM of cross section membrane of PAC C–UF process (a) and PTC C–UF process (b).
were formed by particles/aggerates could not be removed by backwashing 4. Conclusions The comparisons of PTC and PAC in coagulation performance, floc properties and membrane fouling were studied in this work. Several findings had been concluded as follows: 1. Compared with PTC coagulation effluent, PAC coagulation effluent achieved lower residual turbidity. The UV254 and DOC removal both increased as dosage increased. In addition, the maximum removal efficiencies of UV254 and DOC by PTC were higher than those achieved by PAC, respectively. 2. Flocs formed by PTC were larger, stronger and more compact, while flocs formed by PAC were smaller, weaker and looser. However, flocs formed by PAC had better recovery ability compared with those formed by PTC. 3. Membrane fouling caused by PAC C–UF process was a slightly more serious than that by PTC C–UF process. To be specific, the external resistance by PAC was higher compared with PTC, while PTC flocs seemed to be more strongly attached on the membrane surface. Acknowledgments This work was supported by grants from the Chinese National Natural Science Foundation (No. 51278283), the Chinese National Natural Science Foundation for Distinguished Young Scholar (No. 51508308) and Tai Shan Scholar Foundation (No. ts201511003). Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2018.03.057. References [1] Lee S, Ihara M, Yamashita N, Tanaka H. Improvement of virus removal by pilotscale coagulation–ultrafiltration process for wastewater reclamation: effect of optimization of pH in secondary effluent. Water Res 2017;114:23–30. https: //doi.org/10.1016/j.watres.2017.02.017. [2] Cai W, Liu J, Zhang X, Ng WJ, Liu Y. Generation of dissolved organic matter and byproducts from activated sludge during contact with sodium hypochlorite and its implications to on-line chemical cleaning in MBR. Water Res 2016;104:44–52. https://doi.org/10.1016/j.watres.2016.07.065. [3] Huang H, Schwab K, Jacangelo JG. Pretreatment for low pressure membranes in water treatment: a review. Environ Sci Technol 2009;43:3011–19. doi:10.1021/ es802473r.
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Journal of the Taiwan Institute of Chemical Engineers 0 0 0 (2018) 1–8
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Effects of polytitanium chloride and polyaluminum chloride pre-treatment on ultrafiltration process: Floc properties and membrane fouling Xin Huang a,b, Baoyu Gao a,∗, Qinyan Yue a, Yan Wang a, Qian Li a a
School of Environmental Science and Engineering, Shandong University, No. 27 Shanda South Road, Jinan 250100, Shandong, People’s Republic of China Key Laboratory of Drinking Water Science and Technology, Research Center For Eco-environment, Chinese Academy of Science, Beijing 100085, People’s Republic of China
b
a r t i c l e
i n f o
Article history: Received 6 November 2017 Revised 30 March 2018 Accepted 31 March 2018 Available online xxx Keywords: Coagulation–ultrafiltration Membrane fouling Floc properties Polytitanium chloride
a b s t r a c t This study compared the membrane fouling of polytitanium chloride (PTC) and polyaluminum chloride (PAC) in coagulation–ultrafiltration (C–UF) process in real water treatment. According to pre-coagulation experiment, the chosen dosage for PTC and PAC was 18 mg/L as Ti and 8 mg/L as Al, respectively. Floc properties also comparatively studied to further analyze the membrane fouling. PTC formed larger and stronger flocs with higher dimension fractal dimension (Df ) value compared with those formed by PAC. Moreover, PTC C–UF achieved less serious decline of permeate flux during the filtration process. The external membrane resistance of PTC C–UF process was lower than that of PAC C–UF process, while the cake layer of PTC seemed to be more strongly attached on the membrane. For the total internal membrane resistance, PAC achieved lower degree compared with PTC. However, the irreversible membrane resistance of PAC was more serious than that of PTC. Thus, PTC was an effective coagulant for C–UF process. © 2018 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Membrane technology, especially low-pressure membrane including microfiltration and ultrafiltration has been a promising process for drinking water treatment [1]. Membrane fouling has been one of the most serious obstacles which impedes the development of membrane technology [2]. Some pretreatments could be integrated with membrane filtration which contribute to the reducing membrane fouling degree and enhancing the removal of contaminates in water [3]. Coagulation has been one of the most commonly used pretreatment technologies due to its high performance and low cost to remove not only colloids and particles but also natural organic matter (NOM) [4]. The performance of coagulation–ultrafiltration (C–UF) hybrid process usually depends on the features of coagulation, such as coagulant type, dosage and method of mixing and so on [5–7]. Previous study suggested that membrane fouling could be controlled by inorganic coagulant, while it would be aggravated by polymeric organic coagulant [6]. They also suggested that polyaluminum chloride (PAC) exhibited the good performance in reduction of membrane fouling.
∗
Corresponding author. E-mail addresses: [email protected], [email protected], [email protected] (B. Gao).
In addition, PAC has been one of the most popular coagulants in the application of water treatment due to its high performance and low cost for recent decades. However, residual aluminum in drinking water still imposes some potential neuropathological hazards on human being and other organisms [13,14]. In addition, there is another environmental problem caused by aluminum salts. Large amount of sludge produced after coagulation is difficult to deal with and may result in secondary pollution. Recently, titanium salts have been widely studied in coagulation and flocculation due to its well coagulation performance and sludge reuse possibility [15–17]. Shon et al. have first found that sludge of TiCl4 coagulation could be used to produce photocatalyst TiO2 which even performed better photoactivity than commercial P-25 TiO2 [17]. Afterwards, research on Ti-based coagulants gradually increase. To avoid the lower pH of the titanium coagulation effluent, Zhao et al. have successfully synthesized inorganic polymeric titanium coagulantpolytitanium chloride (PTC) which has been proved to be an effective and promising coagulant for water treatment [16]. Zhao et al. have also compared coagulation performance and floc properties of PAC and PTC with various basic values in humic–kaolin water treatment [15]. Floc properties, including floc size, floc strength and floc density also believed to significantly affect membrane fouling, especially the cake layer. Cake resistance has also been proved to be the primary account for the permeate flux decline and membrane
https://doi.org/10.1016/j.jtice.2018.03.057 1876-1070/© 2018 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
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fouling [8,9]. Waite et al. have found that larger floc with loose structure would form a more permeable cake layer [10]. Yu et al. proved floc breakage and re-growth properties also influence the trans-membrane pressure [8]. The membrane fouling or resistance would significantly depend on the physicochemical properties and structure characters of aggregates in the feed water [11,12]. Floc properties is much related to coagulation mechanism which will also affect the membrane fouling. Moreover, floc strength and recoverability are also due to it is inevitable for flocs to break into small particles which generally reduce the solid/liquid separation efficiency and may also plug membrane pores. Previous study also found that compared to flocs formed by PAC, flocs formed by PTC are larger, stronger and difficult to recover after exposure to high shear rate. However, the application of PTC in coagulation–ultrafiltration (C–UF) process has never been studied before. Therefore, this study compared the performance of PAC and PTC in the application of C–UF process for real water treatment. Coagulation performance and floc properties including floc size, floc size distribution, floc breakage and re-growth factor and floc fractal dimension of PAC and PTC were comparatively investigated. Membrane filtration was conducted by the coagulation effluent without sedimentation. Membrane fouling was also divided into external fouling and internal fouling to further compare the effect of PAC and PTC coagulation on the ultrafiltration membrane resistance. 2. Materials and methods 2.1. Coagulants In this research, PAC/PTC was prepared by slow injection of Na2 CO3 / NaOH solution into AlCl3 /TiCl4 solution (20 %wt) at room temperature. The amount of Na2 CO3 or NaOH varied with the target basicity value ([OH]/[Ti] molar ratio, B value). PAC and PTC with B value of 2.0 and 0.5 were chosen, respectively. 2.2. Test water Test water used in this study was collected from the Queshan Reservoir (located in Jinan, China). The properties of the test water were shown as follows: UV254 = 0.045 ± 0.002, DOC = 2.5 ± 0.2 mg/L, pH = 8.40 ± 0.06, zeta potential = −14 ± 0.3 mV, turbidity = 2.5 ± 0.8 NTU. 2.3. Jar test Jar test was conducted by a porogram-controlled jar test apparatus (ZR4-6, Zhongrun Water Industry Technology Development Co. Ltd., China) which has six 1.5 L square beakers. In each beaker, there is a 50 mm × 40 mm flat paddle impeller. Firstly, 1 L of test water was filled in each beaker. Pre-determined amount of coagulant was added and simultaneously the coagulation program began. Then, each beaker of test water was first stirred at 200 rpm for 1.5 min, followed by 15 min of slow stirring at 40 rpm, and finally settled for 30 min or 20 min for PAC or PTC, respectively. About 150 mL of water samples were collected from 2.0 cm below the solution surface for following measurements immediately at the end of the coagulation process. Unfiltered water samples were applied for turbidity (2100P turbidimeter (Hach, USA)) and zeta potential (Zetasizer 30 0 0HSa (Malvern Instruments, UK)). Filtered water samples (through 0.45 μm glass filter paper) were applied for UV245 absorbance and DOC measurement using Precision Scientific Instrument (Co. Ltd., Shanghai, China) and TOCVCPH (Shimadzu, Japan), respectively. The effluent organic matter was also characterized by luminescence spectrometry (F-4500
FL spectrophotometer, Hitachi, Japan) and performance size exclusion chromatography (HPSEC) method and detected by high performance liquid chromatography system (Waters 1525, Waters, USA). 2.4. Floc formation, breakage and re-growth Different from the jar test program, after the 15 min slow stirring of 40 rpm, the suspension was exposed to a high shear rate (200 rpm) for 5 min, and then another 15 min of slow stirring at 40 rpm was re-introduced for floc to re-grow. Mastersizer20 0 0 (Malvern, UK) was used to measure the dynamic floc size during the whole process. The suspension was monitored by drawing water through the test cell of Mastersizer and back into the jar by a peristaltic pump (LEAD-1, Longer Precision Pump, China) on a return tube with 5 mm internal diameter at a flow rate of 2.0 L/h. The inflow and outflow tubes were placed opposite one another at a depth just above the impeller in the holding ports. Size distribution measurements were taken every 0.5 min and logged onto computer. Floc size, breakage and re-growth factor, fractal dimension were calculated according to Ref. [18] and more details could be found in SI information. 2.5. Coagulation–ultrafiltration procedure To compare the membrane fouling degree of PAC and PTC C– UF process, ultrafiltration experiments were conducted using a 300 mL capacity stirring dead-end batch unit. There was an opening designed for pre-treated samples addition at the top of the cell. The effluent after coagulation without sedimentation was gently transferred from coagulation tank to the dead-end filtration unit and filtered through ultrafiltration membrane. Slow agitation was applied to make a uniform suspension. A constant pressure at 0.80 MPa was maintained by nitrogen during the entire filtration process. In addition, cumulative permeate mass was recorded by an electronic balance (MSU5201S-0 0 0-D0; Sartorius AG, Germany) at every 10 s. The UF membrane (Mosu, China) used in this study was polyethersulfone (PES) flat sheet membrane with molecular weight cutoff (MWCO) of 100 kDa (purchased from EMD Millipore Corporation (CAT. NO. PLHK07610). In addition, the membrane had been soaked in deionized water at least 24 h before use. Modified fouling index (MFI) was calculated and specific method could be found in SI Section 2. The initial membrane flux was in a range of 429–435 L/m2 h. In order to analysis the membrane resistance in detail, membrane resistance was specifically divided into external fouling resistance (Ref , also be commonly called cake layer resistance) and internal resistance (Rif ) according to Ref. [19]. Ref was distributed into loosely and strongly attached external fouling (Ref-l and Ref-s ) according to its immunity to shear force. Meanwhile, internal fouling resistance was expressed as reversible and irreversible internal fouling resistance (Rif-r and Rirr ) based on backwash treatment. The calculation of fouling resistance is described in SI Section 2. 3. Results and discussion 3.1. Pre-coagulation by PTC and PAC The effect of coagulant dosage on coagulation performance of PAC and PTC was comparatively studied in real water treatment and results are shown in Fig. 1. To obtain a further view of coagulation behavior of PAC and PTC, wide dosage ranges (2– 30 mg/L as Al for PAC and 2–90 mg/L as Ti for PTC) were adopted in this study. Residual turbidity of PTC firstly gradually decreased and then sharply increased as Ti dosage increased. Moreover, residual turbidity of PAC remained around 0.5 NTU in
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the studied dosage range. It could be noted that residual turbidity of PAC was lower than that of PTC. UV254 and DOC removal efficiencies of PAC and PTC both first increased and then slightly decreased with dosage increased. Initially, UV254 and DOC removal efficiencies of PTC were lower than those of PAC. Afterwards, when dosage was over 24 mg/L, UV254 and DOC removal efficiency of PAC slightly decreased as dosage increased, and those of PTC still gradually increased to 69.33% and 36.24%, respectively. The maximum UV254 and DOC removal efficiencies
of PAC were only 58.66% and 36.24%, respectively. The effluent of solution pH is shown in Fig. 1(e). It could be noted that the effluent pH of PTC coagulation was lower than that of PAC. As for PAC and PTC dosage was 8 and 18 mg/L, respectively, the properties of effluent organic matters were characterized by EEM and HPSEC to evaluated their species and molecular weight and shown in Fig. 2. The EEM spectrogram was divided into five parts according to Ref. [20] and each part represent a kind of organic
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matter. It could be noted that the proteins are the major species in effluent obtained after PAC and PTC coagulation and the intensity of PAC coagulation effluent was higher than that of PTC which meant that the PAC coagulation effluent contained more organic matter. The molecular weight of PAC and PTC coagulation was mainly ranged from 10 0 0–80 0 0 Da. Similarly, the UV absorbance of PAC coagulation effluent was stronger than that of PTC. Zeta potentials of PAC and PTC both gradually increased as dosage increased. Zeta potential of PAC was higher than that of PTC. That indicated that the charge neutralization ability of PAC was better than that of PTC, which was consistent with result of Zhao et al. [15]. However, the higher charge neutralization ability of PAC did not result in higher UV254 and DOC removal. It could be inferred that other coagulation mechanism including sweep, complexion, and adsorption and bridge effect may also played important roles in PTC coagulation. It could be noted that residual turbidity of PTC sharply increased when zeta potential increased from negative to positive. That may be caused by that the surface charge of the formed colloids and particles by titanium and pollutants in water was positively charged, and the inter particle repulsion was increased. Therefore, these particles and colloids is difficult to get together to settle down [21]. In the case of PAC, residual turbidity did not show a rapid increased after isoelectric point. Similar result was found by Yang et al. that PAC (b = 2.0) exhibited better turbidity removal and residual turbidity did not reached minimum at isoelectric point and remained low value even if zeta potential turned
to positive [22]. That was to say surface adsorption and sweep coagulation played efficient role in turbidity removal of PAC. As previous study concerned, membrane fouling was lower under lower coagulant dosage rather than higher dosage, and high dosage coagulation pretreatment usually result in irreversible fouling [5]. They also suggested that charge neutralization coagulation performed better than sweep coagulation in alleviating membrane fouling. Thus, on the basis of above consideration and NOM removal performance, 8 mg/L and 18 mg/L were chosen to be applied in following investigation for PAC and PTC, respectively. 3.2. Floc properties of PTC and PAC According to previous study, the decline of low pressure membrane permeation is mainly caused by the deposition and blocking on or inside the membrane [3]. Also, some study suggested not only floc size and structure would affect the cake layer, but also floc strength could make a difference [10]. Thus, floc properties could significantly affect membrane fouling. Floc growth, breakage and regrowth curve of PAC and PTC was monitored by Mastersizer20 0 0 shown in Fig. 3. More specific floc characters are discussed as follows. 3.2.1. Floc size distribution Particle size distributions (PSD) in the steady period of formation stage of PAC and PTC are shown in Fig. 4. The PSD of PAC flocs
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was mainly between 50–10 0 0 μm. As for PTC flocs, PSD was mostly ranging from 40 0–120 0 μm which exhibited a narrower sharp than that of PAC floc. From Fig. 3, the mean size d50 of PTC was larger than that of PTC. The result was consistent with previous study by Zhao et al. [15]. In addition, previous study by Yu et al. found that more electronegative zeta potential was related with a larger floc mean size d50 for the FeCl3 –humic flocs [5]. In this study, the zeta potential value of PTC (18 mg/L) was lower than that of PAC (8 mg/L). Similarly, greater floc d50 value was obtained by PTC floc. Thus, it could be inferred that the PTC primary aggregates could bind more particles and finally formed larger flocs. 3.2.2. Floc breakage and recovery factor The breakage factor (Bf ) and recovery factor (Rf ) of PTC and PAC were calculated by Eqs. (1) and (2) in SI, respectively, and results are shown in Fig. 5. For the breakage factor, PAC flocs attained smaller value compared with PTC flocs. However, PAC flocs had a larger recovery factor than that of PTC flocs. Similar results were obtained by Zhao et al. who compared PAC and PTC with various B values in synthetic humic acid–kaolin water treatment [15]. They also suggested that PAC coagulants produced flocs with better recoverability than PTC. In the view of breakage factor, PTC floc seemed to be more resistant to high shear rate. That was to say that the inter-particle bind strength inside the PTC flocs was
0
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Time Fig. 6. Floc fractal dimension (Df ) value of PAC and PTC during the formation, breakage and re-growth stage.
greater, which may lead to a more active attached and hard to removed cake layer on the surface of membrane. For the recovery ability, PTC flocs performed poorly as previous studies proved [23]. That was to say that the PTC broken flocs were difficult to get together after exposure to high shear rate. Previous studies proved that flocs formed by charge neutralization effect usually have better recovery ability after exposure to high shear stress [9,24]. Compared with PAC, PTC attained a more electronegative zeta potential. Thus, PTC floc showed worse recovery ability. 3.2.3. Floc structure In this study, floc structure was evaluated by Df value calculated by Eq. (4) in SI and results are shown in Fig. 6. Floc Df value generally increased as coagulation proceeding. It was consistent with previous studies that floc Df value was greater after breakage [25]. It was obviously that during the entire process, PTC flocs Df values were all higher than those of PAC flocs. Li et al. had found that fractal dimension of flocs formed by sweep effect was higher than those formed by charge neutralization and bridge effects [26]. Therefore, compared with PTC flocs, PAC flocs seemed to have a more open and looser structure. It could be inferred that the density of the cake layer by PTC C–UF process might be much greater than that formed by PAC C–UF process. As mentioned above, the
Please cite this article as: X. Huang et al., Effects of polytitanium chloride and polyaluminum chloride pre-treatment on ultrafiltration process: Floc properties and membrane fouling, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.03.057
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Time (min) Fig. 7. Permeate flux of PAC and PTC coagulation–ultrafiltration process.
cake layer of PTC C–UF process was formed by larger, stronger and more compact flocs and that of PAC C–UF process was formed by smaller, weaker and looser flocs. Previous study suggested that larger, looser and more porous flocs attributed to alleviate the decline of flux permeate [10]. However, how the larger and more compact PTC flocs might affect the membrane fouling would be studied in the following section. 3.3. Membrane fouling of PAC and PTC in C–UF application In the membrane filtration experiments, the coagulation suspension without sedimentation was transferred into membrane module and the permeate flux was monitored by an electrical balance and result is illustrated in Fig. 7 and more experiments conditions (including coagulant dose and pH) were conducted as shown in Figs. S1 and S2. The results in Fig. 7, Figs. S1 and S2 showed that the membrane fouling of PTC coagulated water was only a little slighter than that of PAC coagulated water. The permeate fluxes of PAC and PTC coagulated water both first dramatically decreased and then generally decreased relatively gently. The permeate flux decline of PAC was slightly severer than that of PTC and it also could be perceived from MFI shown in Fig. 7. MFI values of PAC and PTC were 99.04 and 81.88, respectively. That was indicated that the total membrane fouling degree was more serious by PAC coagulated water than PTC coagulated water. Moreover, to specifically analyze membrane fouling of PAC and PTC coagulated water, another experiment had been done as described in Section 2.5 and results are shown in Fig. 8. Similarly, the total resistance of PAC C–UF process (3.01∗ 1011 m−1 ) was slightly higher than that of PTC C–UF process (2.66∗ 1011 m−1 ). More specifically, Ref (sum of Ref-l and Ref-s ) (mainly caused by the cake layer deposited on the membrane surface) of PAC C–UF process was higher compared with those of PTC C–UF process. In addition, it could also be note from Fig. 8 that for PAC, Ref-l is higher than that Ref-s ; meanwhile for PTC, Ref-s occupied larger proportion of the external resistance. When comparing PAC C–UF system and PTC C– UF systems, Ref-l of PAC C–UF process was slightly higher; while for Ref-s , PTC C–UF process attained higher value. As for the internal resistance, the total internal resistance (sum of Rirr and Rif-r ) of PAC was lower than that of PTC. However, to be specific, the irreversible resistance Rirr of PAC was slightly higher than that of PAC. For Rif-r , PTC C–UF process was almost three times higher than that of PAC. That was to say, for the internal fouling, PTC C–UF
Fig. 8. Effect of PAC and PTC coagulation pretreatments on ultrafiltration membrane fouling.
resistance was mostly due to the reversible fouling, while PAC C– UF process mainly produced irreversible membrane fouling which was not good for the membrane application. In a comprehensive view of floc properties to analyze the membrane fouling, especially external membrane fouling, membrane fouling has been proved to be related to floc size and structure. In general, larger and looser flocs lead to lower membrane fouling degree. Other study suggested membrane fouling could not only depend on floc size, but also significantly rely on the ability of floc resist to shear stress [27]. They suggested that floc with good ability to resist shear stress would bring out dramatic decline of permeate flux. However, in this study PAC floc was smaller, weaker and looser while PTC floc was larger, stronger but more compact. Therefore, it was not simply to qualitatively analyze the membrane fouling through the floc size and structure. Nevertheless, these floc properties could explain the external membrane resistance. PTC coagulated water achieved lower total membrane fouling and external resistance. In addition, to subdivide and further compare the external fouling, the loosely attached external fouling of PAC was higher but the strongly attached external fouling of PTC was higher. That may due to the high Df value of PTC floc by which a compact cake layer was formed. For the PAC C– UF process, flocs were looser and did not strongly attach on the membrane. Thus, the Ref-l of PAC C–UF process was slightly higher and Ref-s of PTC–UF process was slightly higher. For the internal resistance, pore blocking and adsorption may be one of the most important factors increased membrane fouling. Wang and Tarabara believed that pore blocking mainly contributed to ultrafiltration membrane flux decline [28]. They also suggested that the blocking mechanism could be interpreted by floc size distribution and surface density of membrane. It could be inferred from Fig. 2 that PAC coagulated water may contained more organic matter which was easy to cause membrane pore block. In the membrane of PTC C– UF process, there were not apparent particles. For the irreversible resistance, previous studies suggested that it was mainly caused by the hydrophilic fraction of NOM [29,30]. It could be inferred that the PTC combined more hydrophilic fraction to form larger floc, thus less of hydrophilic fraction was remained in effluent, resulting in lower Rirr value. In addition, the scanning electron microscope (SEM) images of the membrane cross section (after backwashing) were shown in Fig. 9. It was obviously that some small particles were inside the membrane of PAC C–UF process, while the membrane of PTC C–UF process was relatively smooth which
Please cite this article as: X. Huang et al., Effects of polytitanium chloride and polyaluminum chloride pre-treatment on ultrafiltration process: Floc properties and membrane fouling, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.03.057
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Fig. 9. SEM of cross section membrane of PAC C–UF process (a) and PTC C–UF process (b).
were formed by particles/aggerates could not be removed by backwashing 4. Conclusions The comparisons of PTC and PAC in coagulation performance, floc properties and membrane fouling were studied in this work. Several findings had been concluded as follows: 1. Compared with PTC coagulation effluent, PAC coagulation effluent achieved lower residual turbidity. The UV254 and DOC removal both increased as dosage increased. In addition, the maximum removal efficiencies of UV254 and DOC by PTC were higher than those achieved by PAC, respectively. 2. Flocs formed by PTC were larger, stronger and more compact, while flocs formed by PAC were smaller, weaker and looser. However, flocs formed by PAC had better recovery ability compared with those formed by PTC. 3. Membrane fouling caused by PAC C–UF process was a slightly more serious than that by PTC C–UF process. To be specific, the external resistance by PAC was higher compared with PTC, while PTC flocs seemed to be more strongly attached on the membrane surface. Acknowledgments This work was supported by grants from the Chinese National Natural Science Foundation (No. 51278283), the Chinese National Natural Science Foundation for Distinguished Young Scholar (No. 51508308) and Tai Shan Scholar Foundation (No. ts201511003). Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2018.03.057. References [1] Lee S, Ihara M, Yamashita N, Tanaka H. Improvement of virus removal by pilotscale coagulation–ultrafiltration process for wastewater reclamation: effect of optimization of pH in secondary effluent. Water Res 2017;114:23–30. https: //doi.org/10.1016/j.watres.2017.02.017. [2] Cai W, Liu J, Zhang X, Ng WJ, Liu Y. Generation of dissolved organic matter and byproducts from activated sludge during contact with sodium hypochlorite and its implications to on-line chemical cleaning in MBR. Water Res 2016;104:44–52. https://doi.org/10.1016/j.watres.2016.07.065. [3] Huang H, Schwab K, Jacangelo JG. Pretreatment for low pressure membranes in water treatment: a review. Environ Sci Technol 2009;43:3011–19. doi:10.1021/ es802473r.
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Please cite this article as: X. Huang et al., Effects of polytitanium chloride and polyaluminum chloride pre-treatment on ultrafiltration process: Floc properties and membrane fouling, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.03.057